Method for producing grain-oriented electrical steel sheet

ABSTRACT

A method for producing a grain-oriented electrical steel sheet according to an embodiment of the present invention comprises the steps of: heating a slab satisfying the following formula 1 and including, by wt %, 3.2 to 4.0% of Si, 0.03 to 0.09% of C, 0.015 to 0.040% of A1, 0.04 to 0.15% of Mn, 0.001 to 0.005% of N, 0.01% or less of S (exclusive of 0%), and the balance of Fe and other inevitable impurities; hot rolling the slab to produce a hot rolled sheet; hot band annealing the hot rolled sheet to at a cracking temperature of 900 to 980□ for 30 to 300 seconds; cold rolling the hot rolled sheet, which has completely been hot band annealed, to produce a cold rolled sheet; subjecting the cold rolled sheet to primary recrystallization annealing; and conducting secondary recrystallization annealing for the cold rolled sheet which has completely been primary recrystallization annealed. 
       [Mn]×[S]≤0.0004  [Formula 1]
     (wherein [Mn] and [S] are contents (wt %) of Mn and S in the slab, respectively.)

TECHNICAL FIELD

The present invention relates to a method for producing a grain-oriented electrical steel sheet. More specifically, the present invention relates to a method for producing a grain-oriented electrical steel sheet capable of achieving improvement of both productivity and magnetic properties at the same time.

BACKGROUND ART

A grain-oriented electrical steel sheet is used as an iron core material for electrical devices such as a transformer, a motor, a generator, and other electronic devices, and the like. Since a final product of the grain-oriented electrical steel sheet has an aggregate structure (Goss structure) in which an orientation of a crystal grain is oriented in a {110}<001> direction and has significantly excellent magnetic properties in a rolling direction, the final product of the grain-oriented electrical steel sheet may be used as an iron core material for a transformer, a motor, a generator, and other electronic devices, and the like. In order to reduce energy loss, iron loss is required to be low, and in order to achieve miniaturization of the generator, a magnetic flux density is required to be high.

The iron loss of the grain-oriented electrical steel sheet is divided into hysteresis loss and eddy-current loss. In order to reduce the eddy-current loss, it is necessary to reduce a sheet thickness or to increase an inherent specific resistance. As one of specific methods to increase the inherent specific resistance, particularly, smooth production of grain-oriented electrical steel sheet products containing high Si content is a solution to overcome for development of high-end standard products.

In general, as the Si content of the grain-oriented electrical steel sheet is increased, the inherent specific resistance of the product is increased to lower iron loss, and thus it is possible to produce a high-end product, but it is necessary to overcome problems regarding productivity such as a decrease in yielding percentage due to occurrence of fracture during rolling, and the like.

In particular, the grain-oriented electrical steel sheet of a slab low-temperature heating method requires a cold rolling reduction ratio in which an optimum reduction ratio for securing magnetic properties is high as compared to a high temperature heating method. To this end, it is required to increase a thickness of a hot rolled sheet, thereby increasing frequency of fracture during cold rolling. Further, a high Si-containing material has a deteriorated cold rolling property due to an increased brittleness. Thus, in order to produce a high Si-containing grain-oriented electrical steel sheet product by the low-temperature heating method, a technique for reducing the occurrence of fracture during cold rolling is further required. To this end, several methods have been tried to improve the cold rolling property of high Si-containing material and to improve industrial productivity.

One of methods for improving the cold rolling property to solve this problem may include a method for improving quality of a rolled edge portion, and a method for reducing occurrence of edge crack by processing a processed surface neatly after trimming the rolled edge portion. There are a method for performing trimming at a high temperature, and a method for reducing non-uniformness of the edge portion in hot rolling.

In addition, when the two-pass rolling is started, a number of fractures occur during rolled winding, and thus a method for optimizing a 1-pass cold rolling rate that secures ductility when two-pass rolling is started has been proposed. However, this suggested method is not a method for improving inherent characteristics of a material, and thus there is a limit to improvement. Even if conventional methods are applied, it is not possible to fundamentally solve the fracture due to the occurrence of the edge crack caused by inherent characteristics of the high Si steel sheet.

A microstructure of the grain-oriented electrical steel sheet prior to cold rolling has a pearlite bainite ferrite phase mixed therein. After hot band annealing, decarburization of a surface, particularly, the edge portion is locally generated, resulting in a ferrite single phase in which no transformation phase such as pearlite or bainite or martensite is present, and grain growth occurs depending on an annealing temperature.

At the time of the hot band annealing, all the edge portions where heating is concentrated when increasing a temperature so as to increase a sheet temperature to a high temperature at a heating zone particularly become the ferrite phase by local decarburization, and the grain growth occurs actively, and thus non-uniformity phenomenon, in which crystal grains are coarsened, is generated. In general, when a structure is fine, crack initiation resistance is excellent, the frequency of local occurrence of edge cracks is increased when a coarse grain is present at the edge portion, and a crack length formed during rolling is increased, and thus it is likely to lead sheet fracture.

Meanwhile, when fine and non-uniform precipitates are present before cold rolling, crystal grains are non-uniform in subsequent steps, and eventually incomplete secondary recrystallization or non-uniform secondary recrystallization is formed, resulting in deterioration of product characteristics. Therefore, magnetic properties are secured by controlling a heat treatment temperature in order to maximally employ fine precipitates causing non-uniformity and to precipitate coarse grains.

In other words, in order to secure the magnetic properties of the electrical steel sheet product, it is essential to control fine precipitates through hot band annealing at a sufficiently high temperature. On the other hand, in order to reduce the edge crack causing sheet fracture during cold rolling and to secure productivity, a temperature for hot band annealing should be lowered, which is in a relationship that is contrary to the above-described case.

DISCLOSURE Technical Problem

The present invention has been made in an effort to provide a method for producing a grain-oriented electrical steel sheet.

More specifically, the present invention provides a method for producing a grain-oriented electrical steel sheet capable of achieving improvement of both cold rolling productivity and magnetic properties at the same time, as a high Si-containing grain-oriented electrical steel sheet.

Technical Solution

An exemplary embodiment of the present invention provides a method for producing a grain-oriented electrical steel sheet including: heating a slab satisfying the following formula 1 and including, by wt %, 3.2 to 4.0% of Si, 0.03 to 0.09% of C, 0.015 to 0.040% of A1, 0.04 to 0.15% of Mn, 0.001 to 0.005% of N, 0.01% or less of S (exclusive of 0%), and the balance of Fe and other inevitable impurities; hot rolling the slab to produce a hot rolled sheet; hot band annealing the hot rolled sheet at a cracking temperature of 900 to 980□ for 30 to 300 seconds; cold rolling the hot rolled sheet, which has completely been hot band annealed, to produce a cold rolled sheet; subjecting the cold rolled sheet to primary recrystallization annealing; and conducting secondary recrystallization annealing for the cold rolled sheet which has completely been primary recrystallization annealed.

[Mn]×[S]≤0.0004  [Formula 1]

(wherein [Mn] and [S] are contents (wt %) of Mn and S in the slab, respectively.)

According to an embodiment of the present invention, the slab may further include 0.03 to 0.15 wt % of at least one of Sb and Sn, 0.01 to 0.05 wt % of P, and 0.02 to 0.15 wt % of Cr.

According to an embodiment of the present invention, the slab may further include 0.01 to 0.2 wt % of Cu and 0.01 to 0.05 wt % of Mo.

According to an embodiment of the present invention, the method may further include, after the hot band annealing, cooling the hot rolled sheet at a cooling rate of from 10□/sec to 300□/sec from a starting temperature of 700 to 850□ to 300□.

According to an embodiment of the present invention, the hot rolled sheet may have an elongation of 20% or more after the hot band annealing.

According to an embodiment of the present invention, in the heating of the slab, the slab may be heated at a temperature of 1050 to 1200□.

Advantageous Effects

The grain-oriented electrical steel sheet according to an embodiment of the present invention may have excellent productivity while simultaneously having excellent magnetic properties and productivity of the finally produced grain-oriented electrical steel sheet by precisely controlling contents of Mn and S in the slab and temperature conditions at the time of hot band annealing.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an RD cross-sectional image of an edge portion after hot rolled sheet annealing in Inventive Material 1.

FIG. 2 is an RD cross-sectional image of an edge portion after hot rolled sheet annealing in Inventive Material 4.

MODE FOR INVENTION

Terms such as first, second and third, and the like, are used to describe various portions, components, regions, layers, and/or sections, but are not limited thereto. These terms are only used to distinguish any portion, component, region, layer or section from another portion, component, region, layer or section. Thus, a first portion, component, region, layer or section described below may be referred to as a second portion, component, region, layer or section without departing from the scope of the present invention.

Technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the present invention. Singular forms as used herein include plural forms as long as phrases do not clearly express the opposite meaning thereto. The meaning of “including” used herein specifies particular characteristics, regions, integers, steps, operations, elements and/or components, and does not exclude the presence or absence of other characteristics, regions, integers, steps, operations, elements, and/or components.

When referring to a portion as being “above” or “on” another portion, the portion may be directly positioned above or on the another portion, or may involve another portion interposed therebetween. In contrast, when referring to a portion being “directly above” another portion, another portion is not interposed therebetween.

Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of those skilled in the art to which this invention belongs. Terms that are generally defined in the dictionary are further interpreted as having a meaning consistent with the relevant technical literature and the present invention, and are not to be construed as having ideal or very formal meanings unless defined otherwise.

Further, unless otherwise stated, % means wt %, and 1 ppm is 0.0001 wt %.

In an embodiment of the present invention, further inclusion of an additional element means inclusion by substitution for iron (Fe), which is the balance, in the same content as an additional content of the additional element.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily practice the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

The method for producing a grain-oriented electrical steel sheet according to an embodiment of the present invention includes heating a slab satisfying the following formula 1 and including, by wt %, 3.2 to 4.0% of Si, 0.03 to 0.09% of C, 0.015 to 0.040% of A1, 0.04 to 0.15% of Mn, 0.001 to 0.005% of N, 0.01% or less of S (exclusive of 0%), and the balance of Fe and other inevitable impurities; hot rolling the slab to produce a hot rolled sheet; hot band annealing the hot rolled sheet at a cracking temperature of 900 to 980□ for 30 to 300 seconds; cold rolling the hot rolled sheet, which has completely been hot band annealed, to produce a cold rolled sheet; subjecting the cold rolled sheet to primary recrystallization annealing; and conducting secondary recrystallization annealing for the cold rolled sheet which has completely been primary recrystallization annealed.

Hereinafter, each step will be described in detail.

First, the slab is heated.

The slab includes, by wt %, 3.2 to 4.0% of Si, 0.03 to 0.09% of C, 0.015 to 0.040% of A1, 0.04 to 0.15% of Mn, 0.001 to 0.005% of N, 0.01% or less of S (exclusive of 0%), and the balance of Fe and other inevitable impurities.

Hereinafter, each component of the slab is described.

Si: 3.2 to 4.0 wt %

Silicon (Si) plays a role in lowering loss in core, i.e., the core loss, by increasing specific resistance of a grain-oriented electrical steel sheet material. When a Si content is excessively small, an effect of decreasing specific resistance to lower the iron loss is deteriorated. When the Si content is excessively large, brittleness of the steel is increased, toughness is decreased, and thus the rolling is difficult due to occurrence of sheet fracture during a rolling process, a load is applied to the cold rolling operation, it is not sufficient to a sheet temperature required for pass aging during cold rolling, and formation of secondary recrystallization becomes unstable. In an embodiment of the present invention, even though a relatively large amount of Si is contained, productivity is excellent by precisely controlling contents of Mn and S in the slab while simultaneously precisely controlling temperature conditions at the time of hot band annealing.

C: 0.03 to 0.09 wt %

Carbon (C) is an element that induces formation of an austenite phase. As a carbon (C) content increases, ferrite-austenite phase transformation is activated during a hot rolling process, and an elongated hot rolled strip structure formed during the hot rolling process is increased, and thus ferrite grain growth is suppressed during hot band annealing. In addition, as the C content increases, the elongated hot rolled strip structure, which has higher strength than the ferrite structure, is increased, and by refinement of initial grains of the hot rolled sheet annealed structure, which is a structure at the start of the cold rolling, the aggregate structure after cold rolling is improved, particularly, the goss fraction is increased. This is because an effect of the pass aging during cold rolling becomes large due to residual C present in the steel sheet after the hot band annealing, thereby increasing goss fraction in the primary recrystallized grain. Thus, it is advantageous as the C content is larger. However, in the subsequent primary recrystallization annealing process, decarburization time at the decarburization is prolonged, productivity is impaired, and when the decarburization at the initial stage of heating is not sufficient, primary recrystallized grains become non-uniform, and secondary recrystallization becomes unstable. Thus, the carbon content in the slab is limited to the above-described range. Meanwhile, the finally produced grain-oriented electrical steel sheet may include 0.005 wt % or less of carbon by decarburization in a process such as primary recrystallization annealing, or the like, in a process for producing a grain-oriented electrical steel sheet.

Al: 0.015 to 0.040 wt %

Aluminum (Al) binds with N and precipitates into AlN. However, in annealing in which decarburization and nitrification are simultaneously performed, aluminum forms nitrides in the form of (Al, Si, Mn)N and AlN, which are fine precipitates, and plays a role in inhibiting strong crystal grain growth. Aluminum (Al) that is solubilized more than necessary is required at a predetermined content or more. When the content is excessively small, the number and the volume fraction of the precipitates to be formed are low, and thus an effect of inhibiting the crystal grain growth is not sufficient. When the content is excessively large, the precipitates coarsely grow and the effect of inhibiting the crystal grain growth is deteriorated. Therefore, the content of Al may be adjusted to the above-described range.

Mn: 0.04 to 0.15 wt %

Manganese (Mn), similar to SI, has an effect of lowering core loss by increasing specific resistance to decrease an eddy current loss, and plays a role in forming a crystal grain growth inhibitor by reacting with S present in the steel to form a Mn-based compound or by reacting with the above-described Al and nitrogen ions to form a nitride in the form of (Al, Si, Mn)N. When a content of Mn is excessively small, fine MnS may be non-uniformly precipitated in the hot rolling, and thus magnetic properties may be deteriorated. When the content of Mn is excessively large, an austenite phase transformation ratio is increased during the secondary recrystallization annealing, the goss aggregate structure is severely damaged, and magnetic properties may be abruptly lowered. Therefore, the content of Mn may be adjusted to the above-described range.

N: 0.001 to 0.005 wt %

Nitrogen (N) is an element that reacts with Al, or the like, to form AlN micro precipitates, prevents grain boundary migration to inhibit crystal grain growth, thereby refining a crystal grain size. When these fine AlN grains are appropriately distributed, as described above, after the cold rolling, the structure may be appropriately refined to secure a proper primary recrystallization grain size. However, when the content of AlN is excessive, primary recrystallized grains become excessively fine and non-uniform, and as a result, it is not preferable since driving force causing crystal grain growth at the time of secondary recrystallization is increased due to the fine crystal grains, and crystal grains having an orientation other than Goss may also grow. In addition, when a content of N is excessively large, a content of fine precipitates of AlN precipitated during the hot rolling process is increased to cause non-uniformness, and the hot band annealing is required more severely. Therefore, in the present patent, N is set to 0.005 wt % or less. When nitrification treatment for increasing a nitrogen content is performed between the cold rolling and the secondary recrystallization annealing, it is sufficient that N of the slab is contained in the above-described range.

S: 0.010 wt % or Less

Sulfur (S) is a high segregation element having high solubilization temperature during hot rolling, and thus it is preferable to exclude sulfur as much as possible, but it is a kind of impurities that are inevitable to be contained in steelmaking. In addition, since S forms MnS and affects a primary recrystallization grain size, the S content is preferably limited to 0.010% or less, more preferably 0.006% or less. The lower limit of S may be 0.001 wt %.

When the Mn and S contents are contained to satisfy the following formula 1, after the hot rolling, MnS precipitates are refined, and precipitate in an appropriate amount, and then the precipitates are re-solubilized and precipitated even at a temperature ranging from 900 to 980□ for the hot band annealing of the present invention, thereby achieving uniform distribution control. As a result, occurrence of fracture during cold rolling of the high Si-containing material may be reduced, and uniformity of primary and secondary recrystallized grain sizes is improved, resulting in excellent magnetic properties and uniform product characteristics.

[Mn]×[S]≤0.0004  [Formula 1]

(wherein [Mn] and [S] are contents (wt %) of Mn and S in the slab, respectively.)

Sn, Sb, and P

Phosphorus (P), tin (Sn), and antimony (Sb) may play an auxiliary role in suppressing crystal grain growth by segregation in crystal grain boundaries and may have an effect of improving a primary recrystallization aggregate structure. These are effective elements since they have an effect of forming a magnetic flux density stably.

When P is added in a content of 0.01 wt % or more, an effect thereof is exhibited. When an addition content of P exceeds 0.05 wt %, brittleness is strong and it is difficult to perform cold rolling.

When a mixed content of Sn and Sb is 0.03 wt % or more, an effect thereof is exhibited. When the content thereof exceeds 0.15 wt %, the grain boundary segregation effect is excessively strong, and it is difficult to secure a good surface by suppressing formation of a surface oxide layer during decarburization annealing, a decarburization reaction is not uniform, the primary recrystallized grains are non-uniform, and thus final magnetic properties are not stable. In addition, from the viewpoint of mechanical properties, brittleness is increased due to excessive segregation at grain boundaries, which may cause deterioration of rolling properties. Accordingly, the slab may include at least one of Sb and Sn in an individual or combined content of 0.03 to 0.15 wt %. In other words, Sb alone may be included in a content of 0.03 to 0.15 wt %, or Sn alone may be included in a content of 0.03 to 0.15 wt %, or when both of Sb and Sn are included at the same time, a mixed content of Sb and Sn may be 0.03 to 0.15 wt %.

Cr: 0.02 to 0.15 wt %

Chromium (Cr) is an element that promotes oxidation formation. Addition of an appropriate amount of chromium suppresses formation of a dense oxide layer in a surface layer part and helps to form a fine oxide layer in a depth direction. With addition of Sb and Sn, a Cr content in an appropriate range may be added to further facilitate formation of a primary recrystallization with excellent uniformity. By adding Cr, decarburization and sedimentation due to an increase of Sb and Sn contents are delayed to overcome the phenomenon that the primary recrystallized grains are non-uniform, thereby forming primary recrystallized grains having excellent uniformity and enhancing magnetic properties. According to the contents of Sb and Sn, when the Cr content is added in the above-described range, an internal oxide layer is formed deeper, and a rate of sedimentation and decarburization is increased. Therefore, it is possible to overcome the difficulty in controlling the size and ensuring uniformity of the primary recrystallized grains in the simultaneous decarburization and sedimentation process caused by formation of the dense and thin oxide layer due to addition of Sb and Sn. When the Cr content is less than the lower limit, an effect thereof is insufficient, and when the Cr content exceeds the upper limit value, the oxide layer is excessively formed, an effect thereof is decreased, and it is not preferable since the cost is increased due to addition of the expensive alloy.

Cu: 0.01 to 0.2 wt %

Copper (Cu) binds with S, precipitates as CuS, and mainly forms a (Mn, Cu)S form by mixing copper with MnS, thereby inhibiting crystal grain growth. In addition, similar to Mo, Cu forms a large amount of Goss grains having a precise orientation in the structure of a hot rolled surface portion, thereby reducing a crystal grain size after secondary recrystallization and reducing eddy current loss, and thus iron loss of the final product is decreased, and Goss particles having a precise orientation are grown in a large amount, and thus the magnetic flux density is also increased. When an addition content of Cu is excessively small, an effect thereof is not sufficient. When the addition content thereof is excessively large, the precipitate coarsely grows, and the effect of inhibiting crystal grain growth is deteriorated.

Mo: 0.01 to 0.05 wt %

It is known that molybdenum (Mo) causes secondary recrystallization since Goss particles causing secondary recrystallization in grain-oriented electrical steel sheet are generated during hot rolling and remain on a surface portion of specimen even after cold rolling and primary recrystallization heat treatment. When molybdenum (Mo) is added during the hot rolling of the grain-oriented electrical steel sheet, a large amount of Goss grains having a precise orientation are formed in the structure of the hot rolled surface portion, and grains thereof largely remain even after the first recrystallization heat treatment, thereby increasing Goss particles causing secondary recrystallization. Therefore, after the secondary recrystallization, the crystal grain size is decreased and the eddy current loss becomes small, and thus the iron loss of the final product is decreased, and the magnetic flux density is also increased since the Goss particles having the precise orientation are grown in a large amount.

In addition, Mo, similar to Sn, plays an important role in suppressing crystal grain growth by segregation in crystal grain boundary, and plays a role in stably controlling the crystal grain growth to generate secondary recrystallization at high temperature. Therefore, Mo plays a role in growing Goss particles having more accurate orientation to increase the magnetic flux density. Mo is a very effective crystal grain growth inhibiting segregation element since Mo has a relatively large atom size and a very high melting point as 2623□ to obtain a low diffusion rate in iron, thereby well maintaining a segregation effect thereof up to a high temperature.

When a content of Mo is excessively small, an effect of improving magnetic properties is present but the effect is not significant, and an effect of improving the degree of integration of the goss aggregate structure is low, and rather, an effect of compensating crystal grain growth inhibiting power by grains present in the matrix is small, and thus an effect of improving the magnetic properties is not sufficient. On the other hand, when the content of Mo is excessively large, the crystal grain growth inhibiting force is excessively increased, and thus the crystal grain size of the primary recrystallized microstructure is required to be reduced in order to relatively increase a crystal grain growth driving force, and thus it is required to perform decarburization annealing at a low temperature. Therefore, it is not possible to control an appropriate oxide layer, and thus a good surface may not be secured. Therefore, when Mo is further included, Mo may be added in the above-described range.

Ni: 0.03 to 0.1 wt %

Nickel (Ni) is an element that complements the saturation flux density, which is weakened by decrease in magnetic anisotropy according to the increase of Si content, thereby increasing a final magnetic flux density. Ni, similar to C, is an austenite forming element, activates the austenite phase transformation in the hot rolling and in a heat treatment step after hot rolling, thereby bringing an effect of refining a structure. In particular, formation of the goss crystal grain in a sub-surface layer portion is promoted to increase the Goss fraction in the primary recrystallization grain, thereby improving uniformity of the size of the primary recrystallized grain. Therefore, the magnetic flux density of the final product may be increased, and the lower limit of the C content according to the Si content may be lowered by further adding Ni. When the addition amount of Ni is less than the lower limit, an effect thereof is insufficient. When the addition amount of Ni exceeds the upper limit, the addition effect is not significant, and the cost is increased due to addition of the expensive alloy. Therefore, when Ni is further contained, Ni may be added in the above-described range.

Ti: 0.005 wt % or Less

Titanium (Ti) is a strong nitrite forming element, and becomes TiN in a prestep of hot rolling to lower the N content, and is fine precipitated to make crystal grains non-uniform and make secondary recrystallization unstable, and thus a content of titanium is limited to 0.005 wt % or less.

The slab having the above-described composition is heated. The slab may be heated at a low temperature of 1200□ or less, more specifically, at a temperature of 1150□ or less to partially solubilize the precipitate. When a heating temperature of the slab is increased, the production cost of the steel sheet is increased, a heating furnace may be repaired by melting a surface portion of the slab, and the lifetime of the heating furnace may be shortened. Further, when the slab is heated to a temperature of 1050 to 1200□, a columnar structure of the slab is prevented from being coarsely grown, thereby preventing the occurrence of cracks in the width direction of the sheet in the subsequent hot rolling step, resulting in improvement of yielding percentage.

Next, the slab is hot rolled to produce a hot rolled sheet. A hot rolling temperature is not limited, and in an embodiment, hot rolling may be terminated at 950□ or less. Then, water cooling may be subjected to perform winding at 600□ or less. The hot rolled sheet having a thickness of 2.0 to 3.5 mm may be produced by hot rolling.

In the hot rolled sheet which has completely been hot rolled, a columnar structure and an isometric crystal structure which are slab structures are elongated in a hot rolling direction and present non-uniformly. At the same time, coarse precipitates and carbides that are present in the slab are non-uniformly present in the grain and grain boundaries of the hot rolled microstructure. These non-uniform and coarse microstructures, precipitates, carbides, and the like, deteriorate rolling properties of the material during cold rolling, which is a subsequent process, and further cause frequent sheet fracture during rolling. Therefore, it is important that a material which has completely been hot rolled performs hot band annealing so as to have a uniform microstructure and fine and uniformly distributed precipitates.

Next, the hot rolled sheet is subjected to hot band annealing. The hot band annealing may be performed at a cracking temperature of 900 to 980□ for 30 to 300 seconds. The hot band annealing may include a first temperature raising step and a second temperature raising step before reaching the cracking temperature.

Here, the first temperature raising step means a step of temperature raising the hot rolled sheet to 750 to 850□, and the second temperature raising step means a step of temperature raising the hot rolled sheet which has completely been subjected to the first temperature raising step up to the cracking temperature of the cracking step. Specifically, the first temperature raising step is a step of increasing the hot rolled sheet, which has completely been subjected to the hot rolling, up to 750 to 850□. The second temperature raising step is a step of increasing the hot rolled sheet which has completely been subjected to the first temperature raising step, that is, the hot rolled sheet heated up to 750 to 850□, to the cracking temperature in the cracking step. A temperature raising rate (t₁) in the first temperature raising step may be 5 to 45□/sec. When a temperature raising rate t₁ of the first temperature raising step is excessively fast, the number of edge cracks occurring at the edge portion of the cold rolled sheet may be rapidly increased. A temperature raising rate (t₂) in the second temperature raising step may be 1 to 6□/sec. When a temperature raising rate t₂ of the second temperature raising step is excessively fast, the number of edge cracks occurring at the edge portion of the cold rolled sheet may be rapidly increased.

The cracking temperature may be 900 to 980□ and the annealing time, that is, residence time may be 30 to 300 seconds. Through precisely controlled cracking temperature and annealing time, rolling properties in the cold rolling process may be improved, and magnetic properties of the finally produced grain-oriented electrical steel sheet are also improved.

After the hot band annealing, the hot rolled sheet may be cooled at a cooling rate of from 10□/sec to 300□/sec from a starting temperature of 700 to 850□ to 300□. When the cooling rate is excessively low, carbides precipitate and the primary recrystallization aggregate structure is deteriorated, which adversely affects magnetic properties. When the cooling rate is excessively high, stress may remain in the material, for example, a sheet shape may be distorted during the cooling, and the like, and a very slight transformation phase such as martensite or remaining austenite is left in a large amount, and thus rolling characteristics may be deteriorated during cold rolling.

The hot rolled sheet which has completely been hot band annealed has a high elongation, and thus rolling properties in the cold rolling step are improved. Here, the elongation means an elongation obtained in a tensile test after the hot rolled sheet is subjected to tensile specimen processing in accordance with JIS13B standard.

Next, the hot rolled sheet is subjected cold rolling to produce a cold rolled sheet. The cold rolling may be performed to produce a cold rolled sheet having a thickness of 0.15 mm to 0.35 mm by single cold rolling, a plurality of cold rolling or the plurality of cold rolling including an intermediate annealing using a reverse rolling mill or a tandem rolling mill. Further, warm rolling in which the temperature of the steel sheet is maintained at 100□ or more during cold rolling may be performed. In addition, a final reduction rate through cold rolling may be from 50 to 95%.

As described above in an embodiment of the present invention, since hardness of the hot rolled sheet after the hot band annealing step is low and a work hardening index is low, the number of edge cracks formed at the end portion in the thickness direction of the cold rolled sheet in the cold rolling step may be reduced. In an embodiment of the present invention, the edge crack means a crack having a depth of 5 mm or more present at an end portion (edge portion) in the thickness direction of the cold rolled sheet after cold rolling. Specifically, four or less of edge cracks per 50 cm may occur in a length direction of the cold rolled sheet.

Next, the cold rolled sheet which has completely been cold rolled is subjected to primary recrystallization annealing. In the primary recrystallization annealing step, primary recrystallization in which a core of the goss crystal grain is produced is generated. Decarburization and sedimentation of the steel sheet may be performed during the primary recrystallization annealing process. The first recrystallization annealing may be performed in a mixed gas atmosphere of steam, hydrogen and ammonia for decarburization and sedimentation. The first recrystallization annealing may be performed at a temperature of 800□ to 900□ and a dew point temperature of 50□ to 70□ for decarburization. When the temperature exceeds 900□, recrystallized grains are coarsely grown and the crystal growth driving force is reduced, and thus stable secondary recrystallization is not formed. Further, the annealing time is not a serious problem for exerting the effect of the present invention, but it is preferable to treat the annealing within generally 5 minutes in consideration of productivity.

In formation of nitrides such as (Al, Si, Mn)N and AlN, and the like, which are main precipitates, by introducing nitrogen ions into the steel sheet using ammonia gas for nitrification, the use of any one method of a method for performing sedimentation after decarburization and recrystallization are completed, a method for performing decarburization and sedimentation at the same time, or a method for performing sedimentation first and then performing decarburization and annealing has no problem in exerting the effect of the present invention.

Next, the cold rolled sheet which has completely been primary recrystallization annealed is subjected to secondary recrystallization annealing. The secondary recrystallization annealing is performed to form a {110}<001> aggregate structure in which a {110} plane of the steel sheet is parallel to a rolling surface and a <001> direction is parallel to a rolling direction. Here, an annealing separator is applied to the cold rolled sheet which has completely been primary recrystallization annealed, and secondary recrystallization annealing may be performed thereon. Here, the annealing separator is not particularly limited, and an annealing separator including MgO as a main component may be used.

In the secondary recrystallization annealing step, the {110}<001> aggregate structure is formed by secondary recrystallization, a glassy film is formed by a reaction of MgO with the oxide layer on the surface formed through the primary recrystallization annealing heat treatment to impart insulating property, and impurities that impair magnetic characteristics are removed. In the second recrystallization annealing, at a temperature raising section before the second recrystallization is generated, the second recrystallization may be well developed by using a mixed gas of nitrogen and hydrogen to protect the nitride, which is a grain growth inhibitor, and after the second recrystallization has been completed, any method of using a 100% hydrogen atmosphere or a mixed atmosphere of nitrogen and hydrogen has no problem in exerting the effect of the present invention, and the impurities are removed by maintaining the above-described atmosphere for a long period of time.

Thereafter, an insulating film may be formed on a surface of the grain-oriented electrical steel sheet or a magnetic domain refining treatment may be performed, if necessary. In an embodiment of the present invention, the alloy component of the grain-oriented electrical steel sheet refers to a base steel sheet excluding a coating layer such as an insulating coating, or the like.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, these Examples are provided only for illustrating the present invention, and the present invention is not limited thereto.

Example 1

The slab including the components in Table 1 and Table 2, the balance of Fe and inevitably incorporated impurities was heated at 1180□ for 210 minutes and then hot rolled to a thickness of 2.3 m.

The hot rolled sheet was subjected to hot band annealing at a temperature and time according to conditions shown in Table 3 below, cooled up to 760□, quenched in water, and pickled. The hot rolled and annealed sheet was processed according to JIS-13B standard and subjected to a tensile test to measure elongation. Results thereof are summarized in Table 3. A case where the elongation was 20% or more was marked as excellent, and a case where the elongation was less than 20% was marked as defective. FIG. 1 is an RD cross-sectional image of an edge portion after hot rolled sheet annealing in Inventive Material 1, and FIG. 2 is an RD cross-sectional image of an edge portion after hot rolled sheet annealing in Inventive Material 4. As shown in FIGS. 1 and 2, it could be confirmed that crystal grains were uniformly generated when annealing was performed at an appropriate annealing temperature for hot rolled sheet. On the other hand, it could be confirmed that in Comparative Material 4, the crystal grains were generated non-uniformly.

The hot rolled and annealed sheet was subjected to cold rolling once to a thickness of 0.23 mm. The cold rolled sheet was maintained at a temperature of about 860□ in a mixed gas atmosphere of humid hydrogen, nitrogen, and ammonia for 180 seconds to perform primary recrystallization annealing including decarburization and nitrification at the same time so that the carbon content was 50 ppm or less and the nitrogen content was 200 ppm.

This steel sheet was applied with MgO as an annealing separator and finally annealed in a coil phase. The final annealing was performed under a mixed atmosphere of 25 vol % nitrogen and 75 vol % hydrogen until 1200□, and after reaching 1200□, the steel sheet was maintained in a 100 vol % hydrogen atmosphere for 10 hours or more, followed by furnace cooling.

Iron loss and magnetic flux density were measured using a single sheet measurement method, wherein the iron loss was measured until magnetization from 50 Hz to 1.7 Tesla, and the magnitude of magnetic flux density (Tesla) induced under a magnetic field of 800 A/m was measured.

TABLE 1 C S N DELETED- DELETED- DELETED- (wt %) TEXTS Si Mn TEXTS Al TEXTS Mn × S Inventive 0.063 3.43 0.1 0.004 0.03 0.004 0.0004 Material 1 Inventive 0.064 3.46 0.075 0.004 0.03 0.004 0.0003 Material 2 Inventive 0.065 3.46 0.075 0.004 0.03 0.004 0.0003 Material 3 Inventive 0.067 3.5 0.05 0.008 0.03 0.004 0.0004 Material 4 Inventive 0.063 3.43 0.1 0.004 0.03 0.004 0.0004 Material 5 Inventive 0.068 3.48 0.05 0.004 0.03 0.004 0.0002 Material 6 Inventive 0.064 3.46 0.075 0.004 0.03 0.004 0.0003 Material 7 Inventive 0.065 3.43 0.1 0.004 0.03 0.004 0.0004 Material 8 Inventive 0.065 3.43 0.05 0.004 0.03 0.004 0.0002 Material 9 Comparative 0.063 3.45 0.15 0.004 0.03 0.005 0.0006 Material 1 Comparative 0.067 3.49 0.1 0.008 0.03 0.005 0.0008 Material 2 Comparative 0.065 3.44 0.1 0.004 0.03 0.005 0.0004 Material 3 Comparative 0.066 3.48 0.1 0.004 0.03 0.005 0.0004 Material 4 Comparative 0.064 3.47 0.1 0.004 0.03 0.004 0.0004 Material 5 Comparative 0.065 3.43 0.075 0.004 0.03 0.004 0.0003 Material 6

TABLE 2 P DELETED- (wt %) Sn Sb TEXTS Cr Cu Mo Inventive 0.06 0 0.02 0.05 0 0 Material 1 Inventive 0.06 0 0.02 0.05 0 0 Material 2 Inventive 0.06 0 0.02 0.05 0 0 Material 3 Inventive 0.06 0 0.02 0.05 0 0 Material 4 Inventive 0.06 0 0.02 0.05 0.05 0 Material 5 Inventive 0.06 0 0.02 0.05 0.2 0 Material 6 Inventive 0.05 0.04 0.02 0.05 0 0 Material 7 Inventive 0.06 0.02 0.02 0.05 0 0 Material 8 Inventive 0.06 0 0.02 0.05 0.2 0.03 Material 9 Comparative 0.06 0 0.02 0.05 0 0 Material 1 Comparative 0.06 0 0.02 0.05 0 0 Material 2 Comparative 0.06 0 0.02 0.05 0 0 Material 3 Comparative 0.06 0 0.02 0.05 0 0 Material 4 Comparative 0.06 0 0.02 0.05 0 0 Material 5 Comparative 0.08 0.1 0.02 0.05 0 0 Material 6

TABLE 3 Hot rolled sheet annealing Magnetic condition properties Annealing Residence Hot rolled B₈ (T) temperature time sheet DELETED- W_(15/50) (° C.) (sec) elongation TEXTS (W/kg) Note Inventive 900 180 Excellent 1.933 0.766 — Material 1 Inventive 980 180 Excellent 1.937 0.783 — Material 2 Inventive 980 40 Excellent 1.939 0.747 — Material 3 Inventive 980 300 Excellent 1.937 0.777 — Material 4 Inventive 930 180 Excellent 1.94 0.787 — Material 5 Inventive 950 180 Excellent 1.936 0.763 — Material 6 Inventive 950 180 Excellent 1.944 0.738 — Material 7 Inventive 980 180 Excellent 1.941 0.76 — Material 8 Inventive 980 180 Excellent 1.944 0.78 — Material 9 Comparative 1000 180 Excellent 1.902 0.894 Non- Material 1 uniform magnetic property Deleted- Texts Comparative 950 180 Excellent 1.899 0.895 Non- Material 2 uniform magnetic property Comparative 880 180 Defective 1.903 0.879 Large Material 3 number of edge cracks occur Comparative 1050 180 Defective 1.906 0.874 Large Material 4 number of edge cracks occur Comparative 980 20 Excellent 1.898 0.879 Non- Material 5 uniform magnetic property Comparative 1000 180 Defective 1.905 0.89 Non- Material 6 uniform magnetic property/ defective surface

As shown in Tables 1 to 3, it could be confirmed that when satisfying all of Formula 1 and the temperature and time for hot band annealing of the present invention, magnetic properties were excellent and the rolling properties were excellent. On the other hand, it could be confirmed that when not partially satisfying Formula 1 and the temperature and time for hot band annealing of the present invention, magnetic properties were deteriorated or the rolling properties were deteriorated, and thus a large number of edge cracks were formed.

Example 2

The slab including the components in Table 4, the balance of Fe and inevitably incorporated impurities was heated at 1180□ for 210 minutes and then hot rolled to a thickness of 2.3 m.

The hot rolled sheet was hot band annealed at a temperature and a time condition shown in Table 5 below, and then when the temperature reached a starting temperature for cooling of 800□, the sheet was subjected to air cooling up to 300□ Deleted Texts by quenching in boiling water at 100□. The hot rolled and annealed sheet was processed according to JIS-13B standard and subjected to a tensile test to measure elongation. Results thereof are summarized in Table 5. A case where the elongation was 20% or more was marked as excellent, and a case where the elongation was less than 20% was marked as defective.

The hot rolled and annealed sheet was subjected to cold rolling to a thickness of 0.23 mm. The cold rolled sheet was maintained at a temperature of about 860□ in a mixed gas atmosphere of humid hydrogen, nitrogen, and ammonia for 180 seconds to perform primary recrystallization annealing including decarburization and nitrification at the same time so that the carbon content was 50 ppm or less and the nitrogen content was 200 ppm.

This steel sheet was applied with MgO as an annealing separator and finally annealed in a coil phase. The final annealing was performed under a mixed atmosphere of 25 vol % nitrogen and 75 vol % hydrogen until 1200□, and after reaching 1200□, the steel sheet was maintained in a 100 vol % hydrogen atmosphere for 10 hours or more, followed by furnace cooling.

TABLE 4 C S N P DELETED- DELETED- DELETED- DELETED- (wt %) TEXTS Si Mn TEXTS Al TEXTS Sn TEXTS Inventive 0.063 3.43 0.1 0.004 0.03 0.004 0.06 0.02 Material 10 Comparative 0.068 3.48 0.05 0.004 0.03 0.004 0.06 0.02 Material 7

TABLE 5 Hot rolled sheet annealing condition Magentic Starting Hot properties Annealing Residence temperature rolled B₈ (T) temperature time for cooling sheet DELETED- W_(15/50) (° C.) (sec) (° C.) elongation TEXTS (W/kg) Note Inventive 950 180 800 Excellent 1.933 0.766 — Material 10 Comparative 1050 180 800 Defective 1.937 0.783 Large Material 7 number of edge cracks occur

As shown in Tables 4 and 5, it could be confirmed that when satisfying all of Formula 1 and the temperature and time for hot band annealing of the present invention, magnetic properties were excellent and the rolling properties were excellent. On the other hand, it could be confirmed that when not partially satisfying Formula 1 and the temperature and time for hot band annealing of the present invention, magnetic properties were deteriorated or the rolling properties were deteriorated, and thus a large number of edge cracks were formed.

It will be understood by those of skilled in the art that the present invention is not limited to these embodiments, but may be formed in various different forms, and various modifications can be made without changing technical idea and essential features of the present invention. Therefore, it should be understood that the above-described embodiments are not restrictive but are illustrative in all aspects. 

1. A method for producing a grain-oriented electrical steel sheet comprising: heating a slab satisfying the following formula 1 and including, by wt %, 3.2 to 4.0% of Si, 0.03 to 0.09% of C, 0.015 to 0.040% of A1, 0.04 to 0.15% of Mn, 0.001 to 0.005% of N, 0.01% or less of S (exclusive of 0%), and the balance of Fe and other inevitable impurities; hot rolling the slab to produce a hot rolled sheet; hot band annealing the hot rolled sheet at a cracking temperature of 900 to 980□ for 30 to 300 seconds; cold rolling the hot rolled sheet, which has completely been hot band annealed, to produce a cold rolled sheet; subjecting the cold rolled sheet to primary recrystallization annealing; and conducting secondary recrystallization annealing for the cold rolled sheet which has completely been primary recrystallization annealed. [Mn]×[S]≤0.0004  [Formula 1] (wherein [Mn] and [S] are contents (wt %) of Mn and S in the slab, respectively.)
 2. The method of claim 1, wherein: the slab further includes 0.03 to 0.15 wt % of at least one of Sb and Sn, 0.01 to 0.05 wt % of P, and 0.02 to 0.15 wt % of Cr.
 3. The method of claim 1, wherein: the slab further includes 0.01 to 0.2 wt % of Cu and 0.01 to 0.05 wt % of Mo.
 4. The method of claim 1, further comprising, after the hot band annealing, cooling the hot rolled sheet at a cooling rate of from 10□/sec to 300□/sec from a starting temperature of 700 to 850□ to 300□.
 5. The method of claim 1, wherein: the hot rolled sheet has an elongation of 20% or more after the hot band annealing.
 6. The method of claim 1, wherein: in the heating of the slab, the slab is heated at a temperature of 1050 to 1200□. 